High Dipole Moment Research Articles

Performance enhancement of organic light-emitting diodes using fluorinated self-assembled monolayer

ABSTRACTIn order to enhance the performance of organic light-emitting diodes, fluorinated self-assembled monolayer (F-SAM) with long alkyl chain was introduced to the device between the anode and hole-transport layer. CF3-terminated F-SAM has a relatively high electric dipole moment, which may affect hole injection at the indium-tin-oxide (ITO) anode. Two kinds of F-SAM with different alkyl chain lengths (F3-SAM and F10-SAM) were formed on the ITO substrate through hydrolysis and dehydration process. Electric dipole moments of these F-SAMs were obtained from a simulation using Gauss View program. A device was made in a structure of ITO (170 nm) / F-SAM / TPD (40 nm) / Alq3 (60 nm) / Al (100 nm). Formation of the SAM was confirmed by contact-angle measurements. It was found that the contact angles for the F3-SAM and F10-SAM are about 84° and 112°, respectively. A device with properly treated self-assembled monolayer at the anode gives an improvement in turn-on voltage, luminance, and efficiency compared to those of the device without SAM. Turn-on voltage was lowered from 4.25 V to 3.50 V in the device with SAM compared to the one without SAM, which indicates a reduction of hole-injection barrier height due to the electric dipole moment of the F-SAM. An external quantum efficiency of the device with F3-SAM was increased by a factor of about 3.5 compared to the one without SAM.

Hydrogenation of fluoroarenes: Direct access to all-cis-(multi)fluorinated cycloalkanes.

All-cis-multifluorinated cycloalkanes exhibit intriguing electronic properties. In particular, they display extremely high dipole moments perpendicular to the aliphatic ring, making them highly desired motifs in material science. Very few such motifs have been prepared, as their syntheses require multistep sequences from diastereoselectively prefunctionalized precursors. Herein we report a synthetic strategy to access these valuable materials via the rhodium-cyclic (alkyl)(amino)carbene (CAAC)-catalyzed hydrogenation of readily available fluorinated arenes in hexane. This route enables the scalable single-step preparation of an abundance of multisubstituted and multifluorinated cycloalkanes, including all-cis-1,2,3,4,5,6-hexafluorocyclohexane as well as cis-configured fluorinated aliphatic heterocycles.

Fabricating high-quality ultra-thin croconic acid film using electric field guidance

A novel method for fabricating ultra-thin croconic acid (CA) films with a very low surface roughness on Si wafers is reported. With a thickness of approximately 20nm and surface roughness of ±2.0nm, the film obtained far exceeds the quality and smoothness of previously reported CA thin films. The film is prepared by applying a high electric field in situ during thermal vapor deposition, promoting alignment of the CA molecules due to their high dipole moment. The result is compared with that of the best-reported film produced via a combination of thermal evaporation at low substrate temperature with subsequent slow heating, to demonstrate the greatly enhanced uniformness of the film. In addition, the film’s ferroelectric behavior is demonstrated using piezoresponse force microscopy measurements. A mechanism for film formation under electric field assisted deposition is proposed.

Non-ideal behavior of binary aqueous mixtures of some urea derivatives and their capacity to induce lysozyme gelation

The urea derivatives, namely, ethylurea (EU), 1,3 dimethylurea (1,3-DMU) and 1,1 diethylurea (1,1-DEU), in the limiting regions of their solubilities in water, and tetramethylurea (TMU) at w≥0.65 were investigated in relation to their capacity of inducing hen egg white lysozyme (HEWL) physical (non-covalent) gelation. Protein transparent gels were generated out of TMU/H2O and 1,1-DEU/H2O, respectively, whereas an intensively turbid gel resulted from sol-gel transition taking place in EU/H2O. Oscillatory rheology revealed distinctions in the gels’ structural and dynamic characteristics. Hydration patterns of the derivatives in solution, sizes of their non-polar domains and supramolecular symmetry features played a central role in their capacity of gel formation and in the gels’ rheological behavior and morphology. Effects on gel characteristics of distinctively positioned ions in the Hofmeister series showed that SCN− disrupted water H-bonding interconnectivity in TMU lysozyme gel, strengthening gel structure, yet maintaining gel transparency. Citrate enhanced system elasticity albeit causing intense turbidity and leading to phase separation. Larger values of the storage modulus, G′, were verified for gels generated from binary mixtures containing urea derivatives with higher dipole moments.

Structure, electronic properties and electronic excitation analyses of Si60[sbnd]Si60 dimer and Si59Al[sbnd]Si59P complex

The electronic properties of nanoclusters, such as Si60 cage, are remarkably changed by substitution of heteroatom such as Al or P. How do such impure Si60 cages behave when brought together to form a complex? To address this, we have performed DFT and TD-DFT based calculations on Si60Si60 dimer and Si59AlSi59P complex. We have explored various electronic properties of Si59AlSi59P complex and compared with those of Si60Si60 dimer. We notice that the bonding along with binding energies of cages in the complex is slightly different than that of dimer. The higher dipole moment and smaller energy gap of the complex compared to dimer suggest its increased polarity and conductivity. The density-of-state curves reveal that the frontier orbitals of the complex are slightly shifted relative to dimer such that their energy gap is decreased. The electronic excitation analyses suggest that the electronic transition in Si59AlSi59P complex takes place via charge transfer (CT) mode, and the strength of this CT interaction is much larger than that of Si60Si60 dimer. This is confirmed by larger oscillator strength, larger distance between the centroid of hole and electron as well as larger Δr index value. We believe that these findings might be useful in the application of Si-nanoclusters in the field of molecular electronics and nanoelectronics.

β‐Trisubstituted “Push–Pull” Porphyrins – Synthesis and Structural, Photophysical, and Electrochemical Redox Properties

A family of tri‐β‐substituted porphyrins, MTPP(CH3)2X (M = 2H, Co, Ni, Cu and Zn; X = NO2 or CHO; TPP = tetraphenylporphyrin), was synthesized and characterized by various spectroscopic techniques. The single‐crystal X‐ray structures of NiTPP(CH3)2CHO and H2TPP(CH3)2NO2 revealed ruffled and saddle conformations of the porphyrin core, respectively. The effects of the β substituents were reflected in bathochromic shifts of the electronic spectral features and positive or negative shifts of the redox potentials. H2TPP(CH3)2NO2 and H2TPP(CH3)2CHO exhibited very high dipole moments (5.76–7.59 D) compared with that of H2TPP (0.052 D) owing to the push–pull effect. These push–pull porphyrins exhibited lower HOMO–LUMO gaps than those of MTPP(NO2) and MTPP. The redox tunability was achieved through the introduction of electron donor and acceptor groups at the porphyrin backbone.

Improved performance of colloidal quantum dot solar cells using high-electric-dipole self-assembled layers

High performance colloidal quantum dot (CQD) solar cells were developed by modifying ZnO electron accepting layers (EALs) using self-assembled monolayers (SAMs) of highly polar molecules. A high molecular dipole moment of −10.07D was achieved by conjugating a strong electron donor, julolidine, to an electron acceptor, a cyanoacetic acid unit, through a thiophene moiety. The energetic properties of ZnO EALs were manipulated with respect to the dipole moment of the modifying molecules. The built-in potential (Vbi) and internal electric field (Eint) of CQD solar cells could thereby be tuned. The power conversion efficiency (PCE) of the SAM modified devices was improved from 3.7% to 12.9% relative to the unmodified devices as a function of molecular dipole moments (from −5.13D to −10.07D). All figures-of-merit of solar cells were improved simultaneously by SAM modification due to enhanced Vbi, Eint, and charge collection efficiency. The PCE of the highly polar molecule modified devices reached 10.89% with a VOC of 0.689V, whereas that of the unmodified devices was 9.65% with a VOC of 0.659V. Notably, the remarkably low energy loss of 0.433eV is achieved in the SAM modified devices.

Quantum chemical assessment of the adsorption behavior of fluorouracil as an anticancer drug on the B36 nanosheet

Interaction of fluorouracil, a conventional anticancer drug, with the B36 nanosheet is investigated systematically using the density functional theory (DFT) method. The results indicate that although fluorouracil weakly interacts with convex and concave sides of nanosheet, it is remarkably adsorbed at the edge of B36 nanosheet through its oxygen atoms. The interactions are also studied in terms of NBO charges and molecular electrostatic potential analysis. It is found that the interaction of this drug molecule with nanosheet significantly alters its electronic nature. The energy gap of the stable systems are reduced which could be applied as a chemical signal. Indeed, it is found that the adsorption of fluorouracil molecule at the edge of the B36 nanosheet is effective in aqueous medium. The relative high dipole moments are estimated for B36-FU systems, exhibiting that these systems could be solubilized or dispersed in polar mediums, like water. The obtained results introduce the B36 nanosheet as an efficient carrier for delivery of fluorouracil in nanomedicine domain.

Increased dielectric permittivity of poly(vinylidene fluoride-co-chlorotrifluoroethylene) nanocomposites by coating BaTiO3 with functional groups owning high bond dipole moment

Polymer nanocomposites with high dielectric permittivity (ε) are urgently demanded for power energy storage. In this research, in order to prepare nanocomposites with strong interfacial interactions and higher ε, a novel surface modification molecule, 3,4,5-trihydroxybenzoic acid (or Gallic acid, GA), was developed. There are two main advantages for selecting GA as the coating molecule. Firstly, the polar groups, such as hydroxyl and carboxyl groups in GA, could form hydrogen-bonding interaction with poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE). Therefore, though thickness of coating layer is only 1.6nm, nanocomposites with better dispersion were prepared by strengthening hydrogen-bonding interaction at polymer-nanoparticle interface. Secondly, functional groups with high bond dipole moment, such as carboxyl groups and phenolic groups, are beneficial to obtain high ε in PVDF-CTFE+BT@GA. ε is 25.6 at 1kHz with 12.9% BT@GA. This is about 2.5 times higher than that of PVDF-CTFE matrix (10.4 at 1kHz). The hypothesis in this research might offer a feasible approach to prepare capacitors with higher ε and higher energy density by regulating bond dipole moment of functional groups in the coating molecules.

Dramatic Effect of Solvent on the Rate of Photobleaching of Organic Pyrrole‐BF2 (BOPHY) Dyes

AbstractThe photochemical stability of two symmetrical pyrrole‐BF2 (BOPHY) dyes has been assessed under continuous exposure to white light. The parent dye, lacking conjugated substituents, is highly stable when illuminated in a nonpolar film for prolonged periods but autocatalysis is observed such that the rate of degradation escalates with increasing exposure time. The rate of photofading is faster in solution and involves intermediate formation of a coloured product. The nature of the solvent exerts a small effect on the rate of bleaching. Under the same conditions, a BOPHY derivative equipped with both extended π‐conjugation and a high internal dipole moment displays a pronounced solvent effect. Here, the rate of photofading varies over a factor of 2 000‐fold on changing the solvent from cyclohexane to toluene. Most of the kinetic data can be explained in terms of polar and neutral resonance forms but, on the basis of NMR spectroscopy, it is concluded that toluene forms a weak complex with the dye that provides protection against photochemical damage.

Electron Excitation of High Dipole Moment Molecules Re-examined

Abstract Emission from high-dipole moment molecules such as HCN allows determination of the density in molecular clouds, and is often considered to trace the “dense” gas available for star formation. We assess the importance of electron excitation in various environments. The ratio of the rate coefficients for electrons and H2 molecules, ≃105 for HCN, yields the requirements for electron excitation to be of practical importance if and , where the numerical factors reflect the critical values and . This indicates that in regions where a large fraction of carbon is ionized, will be large enough to make electron excitation significant. The situation is in general similar for other “high-density tracers,” including HCO+, CN, and CS. But there are significant differences in the critical electron fractional abundance, , defined by the value required for equal effect from collisions with H2 and e−. Electron excitation is, for example, unimportant for CO and C+. Electron excitation may be responsible for the surprisingly large spatial extent of emission from dense gas tracers in some molecular clouds. The enhanced estimates for HCN abundances and HCN/CO and HCN/HCO+ ratios observed in the nuclear regions of luminous galaxies may be in part a result of electron excitation of high dipole moment tracers. The importance of electron excitation will depend on detailed models of the chemistry, which may well be non-steady state and non-static.

Adsorption of polar organic molecules on sediments: Case-study on Callovian-Oxfordian claystone

The release and transport of anthropogenic organic matter through the geosphere is often an environmental criterion of safety. Sedimentary rocks are widely studied in this context as geological barriers for waste management. It is the case of Callovian-Oxfordian claystone (COx), for which several studies report adsorption of anthropogenic organic molecules. In this study, we evaluated and reviewed adsorption data of polar organic molecules on COx claystone. Experiments were performed on raw claystone, decarbonated and clay fractions. Adsorption isotherms were measured with adsorbates of various polarities: adipate, benzoate, ortho-phthalate, succinate, gluconate, oxalate, EDTA, citrate. A significant adsorption was observed for multidentate polycarboxylic acids as evidenced with phthalate, succinate, oxalate, gluconate, EDTA and citrate (Rd = 1.53, 3.52, 8.4, 8.8, 12.4, 54.7 L kg−1 respectively). Multiple linear regression were performed as a statistical analysis to determine the predictors from these adsorption data. A linear correlation between adsorption data (Rd) and dipole moment (μ) of adsorbates was evidenced (R2 = 0.91). Molecules with a high dipole moment, μ(D) > 2.5, displayed a significant adsorption, Rd≫1 L kg−1. A qualitative correlation can be easily estimated using the water/octanol partition coefficient, Pow, of adsorbates (R2 = 0.77). In this case, two opposite trends were distinguished for polar and apolar molecules. The use of organic carbon content in sediments is relevant for predicting adsorption of apolar compounds, log (Pow)>+1. The oxides/clays contents may be relevant regarding polar molecules, log (apparentPow)<-1. The proposed scheme offers a general methodology for investigation of geo-barriers towards heterogeneous organic plumes.

High‐Performance Piezoelectric, Pyroelectric, and Triboelectric Nanogenerators Based on P(VDF‐TrFE) with Controlled Crystallinity and Dipole Alignment

Poly(vinylidenefluoride‐co‐trifluoroethylene) (P(VDF‐TrFE)), as a ferroelectric polymer, offers great promise for energy harvesting for flexible and wearable applications. Here, this paper shows that the choice of solvent used to dissolve the polymer significantly influences its properties in terms of energy harvesting. Indeed, the P(VDF‐TrFE) prepared using a high dipole moment solvent has higher piezoelectric and pyroelectric coefficients and triboelectric property. Such improvements are the result of higher crystallinity and better dipole alignment of the polymer prepared using a higher dipole moment solvent. Finite element method simulations confirm that the higher dipole moment results in higher piezoelectric, pyroelectric, and triboelectric potential distributions. Furthermore, P(VDF‐TrFE)‐based piezoelectric, pyroelectric, and triboelectric nanogenerators (NGs) experimentally validate that the higher dipole moment solvent significantly enhances the power output performance of the NGs; the improvement is about 24% and 82% in output voltage and current, respectively, for piezoelectric NG; about 40% and 35% in output voltage and current, respectively, for pyroelectric NG; and about 65% and 75% in output voltage and current for triboelectric NG. In brief, the approach of using a high dipole moment solvent is very promising for high output P(VDF‐TrFE)‐based wearable NGs.

Marine Diterpenes: Molecular Modeling of Thrombin Inhibitors with Potential Biotechnological Application as an Antithrombotic.

Thrombosis related diseases are among the main causes of death and incapacity in the world. Despite the existence of antithrombotic agents available for therapy, they still present adverse effects like hemorrhagic risks which justify the search for new options. Recently, pachydictyol A, isopachydictyol A, and dichotomanol, three diterpenes isolated from Brazilian marine brown alga Dictyota menstrualis were identified as potent antithrombotic molecules through inhibition of thrombin, a key enzyme of coagulation cascade and a platelet agonist. Due to the biotechnological potential of these marine metabolites, in this work we evaluated their binding mode to thrombin in silico and identified structural features related to the activity in order to characterize their molecular mechanism. According to our theoretical studies including structure-activity relationship and molecular docking analysis, the highest dipole moment, polar surface area, and lowest electronic density of dichotomanol are probably involved in its higher inhibition percentage towards thrombin catalytic activity compared to pachydictyol A and isopachydictyol A. Interestingly, the molecular docking studies also revealed a good shape complementarity of pachydictyol A and isopachydictyol A and interactions with important residues and regions (e.g., H57, S195, W215, G216, and loop-60), which probably justify their thrombin inhibitor effects demonstrated in vitro. Finally, this study explored the structural features and binding mode of these three diterpenes in thrombin which reinforced their potential to be further explored and may help in the design of new antithrombotic agents.

Stability and molecular properties of the boron-nitrogen alternating analogs of azulene and naphthalene: a computational study.

In this work, the spectroscopic information, stability and aromaticity of the boron-nitrogen azulene and naphthalene molecules are provided by the use of CC2 (geometry optimization, dipole moment, UV-vis spectrum calculations) and DFT (vibrational spectrum and NMR calculations) methodologies. One isomer of the investigated boron-nitrogen naphthalene (boroazanaphthalene) and two isomers of boron-nitrogen azulene, 1,3,4,6,8-pentaaza-2,3a,5,7,8a-pentaboraazulene (BN-azulene) and 2,3a,5,7,8a-pentaaza-1,3,4,6,8- pentaboraazulene (NB-azulene), are stable systems. However, these molecules have different properties, i.e., different stability, dipole moment, and aromaticity based on the NICS approach. BN-naphthalene has a high dipole moment magnitude showing high polar character, while naphthalene is apolar. BN- and NB-azulene are weakly polar, while ordinary azulene is highly polar in character. Also, substitution of C atoms by B and N atoms decreases the aromaticity. In the case of NB-azulene, the seven-membered ring has anti-aromaticity behavior while both rings of BN-azulene exhibit aromaticity. We expect that the new theoretical data provided in this work will be useful in identifying and characterizing experimentally the compounds investigated, and in helping our understanding of the chemistry of boron-nitrogen molecules. Graphical abstract Boron-nitrogen alternating analogs of azulene. Spectral distinction between isomers.

Photo-catalytic reduction of oxygenated graphene dispersions for supercapacitor applications

Reduced graphene oxide (rGO) obtained from aqueous graphene oxide (GO) tends to agglomerate with time and hinders the commercial scale applications for high-density energy storage. Here, we report a photo-catalytic reduction of GO dispersions in N-Methyl-2-Pyrrolidone (NMP) under deep UV light (λ ~ 253 nm) for 60 min. The obtained hydrophobic rGO dispersions are electrochemically stable for more than 160 d and exhibit a high Brunauer–Emmet–Teller (BET) surface area of ~260 m2 g−1. The NMP being a dipolar aprotic solvent serves as an electron donor and its high dipole moment enhances the electrochemical stability of rGO. Furthermore, the fabricated supercapacitor exhibits a high specific capacitance, charge retention, energy and power density of ~220 F g−1 (current density of 0.5 A g−1), up to 1000 charging/discharging cycles, 7.32 Wh kg−1 and 130 W kg−1, respectively. The high stability of dispersion and electrochemical performance of synthesized rGO is envisaged for potential applications in high density energy storage and conductive inks for flexible electronics.

Formation of planarized intramolecular charge-transfer state in dichlorotriazinyl-pyrene fluorescent probe: TD-DFT and resonance Raman study

This study is focused on explanation of the remarkable photophysical behaviour of the 1-(4,6-dichloro-1,3,5-triazin-2-yl)-pyrene (PyTC2) compound which has been introduced as a fluorescent polarity probe. This compound exhibits large solvatochromic red-shift of fluorescence emission band while maintaining high fluorescence quantum yield and monoexponential decay kinetics throughout the whole solvent polarity scale. As the semi-empirical calculations reported in the original paper have not revealed any excited state possessing a high dipole moment, it has been suggested that the red-shift originates from planarization of the emitting excited state in polar solvents in contrast to unchanged twisted geometry in non-polar solvents. However, both the extent of the red-shift and the disappearance of the vibronic structure in polar solvents indicate that the emission originates from an excited state with high dipole moment and that the semi-empirical methods may not be sufficient to describe the emitting state of this molecule correctly. Thus, we have performed TD-DFT calculations including the potential energy surface scans. According to these calculations and scans, the emission takes place from a planarized intramolecular charge-transfer (ICT) state. This is in good agreement with all aspects of the observed fluorescence behaviour of PyTC2. Independent experimental evidence for the ICT has been provided by analysis of resonance Raman intensities where bands corresponding to enhanced normal modes residing on triazine and the stretching mode between pyrene and triazine moieties have been identified. The formation of the photoinduced ICT together with easy and inexpensive preparation make this compound and its derivatives candidate as push-pull building blocks for the design of advanced functional materials.

Laboratory Refining of Bleached Softwood Kraft Pulp in Water and a Series of Alcohols of Different Molecular Weights and Polarities: Effects on Swelling and Fiber Length

Typical bleached pinewood kraft pulp from a paper mill was immersed in either water or different alcohols, such as methanol, ethanol, n-propanol, and n-butanol, and then refined in a PFI mill. After refining, changes in the internal fibrillation of the fibres were evaluated by measurements of water retention values (WRV), while fibre shortening was determined by measurements of the average weighted fibre length. The objective of this study was to determine the influence of a liquid used for refining on the principal refining effects such as the internal fibrillation and fiber shortening. The highest increase in the internal fibrillation was observed for the pulp samples refined in water, which has the higher dipole moment than the alcohols. For these samples, WRV increased from 93% to 201%, while the average weighted fiber length was reduced by only 0.6 mm. When the pulp was beaten in n-butanol, which was the least polar liquid among the liquids investigated, liquid retention value increased by only 23.5%, while the average weighted fibre length was reduced by 1.37 mm. These results showed the importance of water in the beating of cellulose fibres and demonstrated that the outcomes of this process depended on the dipole moment of the fiber immersion liquid.

Dense gas in low-metallicity galaxies

Stars form out of the densest parts of molecular clouds. Far-IR emission can be used to estimate the star formation rate (SFR) and high dipole moment molecules, typically HCN, trace the dense gas. A strong correlation exists between HCN and far-IR emission, with the ratio being nearly constant, over a large range of physical scales. A few recent observations have found HCN to be weak with respect to the far-IR and CO in subsolar metallicity (low-Z) objects. We present observations of the Local Group galaxies M 33, IC 10, and NGC 6822 with the IRAM 30 m and NRO 45 m telescopes, greatly improving the sample of low-Z galaxies observed. HCN, HCO+, CS, C2H, and HNC have been detected. Compared to solar metallicity galaxies, the nitrogen-bearing species are weak (HCN, HNC) or not detected (CN, HNCO, N2H+) relative to far-IR or CO emission. HCO+ and C2H emission is normal with respect to CO and far-IR. While 13CO is the usual factor 10 weaker than 12CO, C18O emission was not detected down to very low levels. Including earlier data, we find that the HCN/HCO+ ratio varies with metallicity (O/H) and attribute this to the sharply decreasing nitrogen abundance. The dense gas fraction, traced by the HCN/CO and HCO+/CO ratios, follows the SFR but in the low-Z objects the HCO+ is much easier to measure. Combined with larger and smaller scale measurements, the HCO+ line appears to be an excellent tracer of dense gas and varies linearly with the SFR for both low and high metallicities.

A High Contrast Tri-state Fluorescent Switch: Properties and Applications.

A high contrast tri-state fluorescent switch (FSPTPE) with both emission color change and on/off switching is achieved in a single molecular system by fusing the aggregation-induced emissive tetraphenylethene (TPE) with a molecular switch of spiropyran (SP). In contrast to most of the reported solid-state fluorescent switches, FSPTPE only exists in the amorphous phase in the ring-closed form owing to its highly asymmetric molecular geometry and weak intermolecular interactions, which leads to its grinding-inert stable cyan emission in the solid state. Such an amorphous phase facilitates the fast response of FSPTPE to acidic gases and induces the structural transition from the ring-closed form to ring-open form, accompanied with the "Off" state of the fluorescence. The structural transition leads to a planar molecular conformation and high dipole moment, which further results in strong intermolecular interactions and good crystallinity, so when the acid is added together with a solvent, both the ring-opening reaction and re-crystallization can be triggered to result in an orange emissive state. The reversible control between any two of the three states (cyan/orange/dark) can be achieved with acid/base or mechanical force/solvent treatment. Because of the stable initial state and high color contrast (Δλ=120 nm for cyan/orange switch, dark state ΦF <0.01 %), the fluorescent switch is very promising for applications such as displays, chemical or mechanical sensing, and anti-counterfeiting.